This PLY is too dark! How to improve it?
I think that there is not a Blender-related issue: the vertex colors are the ones the file provide.

Here's what they look like in Meshlab:



And in Blender:



put apart the different viewport shading by seeing the true solid colours in cycles preview or textured mode in blender render, and you'll see no difference:







You can tweak the vertex colors with nodes as liked: 



Note that vertex colors are changed only in the viewport, for rendering purpose, not in the vertex color layer. If you want to store a new, brighter, vertex layer you should:


bake cycles tweaked vertex color to an image texture
convert the texture to a new vertex color layer